Feedback Control Method and System Having Variable Deadband

1. Field of the Invention

The present invention relates to a feedback automatic control method and apparatus. More specifically, the present invention relates to a feedback automatic control method and apparatus adapted for making feedback control such that a control amount of an object being controlled may correspond with a target value.

2. Description of the Prior Art

A feedback automatic control apparatus adapted for opening or closing a gate by controlling the same such that the water level of a dam may be a target value in a hydraulic power generating station, for example, has been hitherto employed.

FIG. 1 is a block diagram of a conventional feedback automatic control apparatus. Referring to FIG. 1, the structure of a conventional feedback automatic control apparatus will be described. A control portion 1 is adapted to set in a first comparator 2 a target value of the water level of a dam and is also adapted to set in a second comparator 5 a dead zone having a predetermined width .beta. contiguous to the target value. The width .beta. of the dead zone is set for discontinuing control for opening/closing a gate, not shown, by deeming that the actual water level has reached the target value, if and when the water level of a dam is within such range. The first comparator 2 is supplied with a detected signal from a water level sensor 4 for detecting the actual water level 3 of the dam. The first comparator 2 compares the target value and the actual water level, thereby to evaluate a deviation value to provide the same to the second comparator 5. The second comparator 5 compares the dead zone as set and the deviation obtained from the first comparator 2, so that an operation command signal is applied to a gate driving circuit 6 so that the deviation may be decreased. Accordingly, the gate driving circuit 6 controls the gate of the dam to open/close the same, thereby to change the flow out amount of the water.

FIG. 2 is a graph showing a change of the control amount in the FIG. 1 conventional feedback automatic control apparatus. Now an operation of the conventional feedback automatic control apparatus will be described. The control portion 1 sets in the second comparator 5a dead zone having a predetermined width .beta. shown in FIG. 2 and provides the first comparator 2 with a target value contiguous to the upper limit of the dead zone. The first comparator 2 compares the water level detected by a water level sensor 4 at the start of control and the target value, thereby to evaluate a deviation. The second comparator 5 compares the deviation supplied from the first comparator 2 and the dead zone, thereby to provide the gate driving circuit 6 with an operation command signal associated with the deviation, if and when the detected water level is not within the range of the dead zone. The gate driving circuit 6 is responsive to the operation command signal to open/close the gate, thereby to adjust the water level of the dam. If and when the water level of the dam is within the range of the width .alpha. of the dead zone, an operation command signal is supplied from the second comparator 5 to the gate driving circuit 6, thereby to discontinue control for opening/closing the gate.

Since a conventional feedback automatic control apparatus is adapted such that a dead zone having a predetermined width .beta. at the start of control is set and the gate is opened/closed such that the opening/closing amount of the gate, i.e. the water level by way of the control amount may be a target value without changing the set dead zone, it follows that opening/closing of the gate is frequently made. More specifically, in the case where the width .beta. of the dead zone as set is narrow, the water level by way of the control amount as detected by the water level sensor 4 comes slightly off the target value and each time comes off the dead zone, with the result that the gate is frequently opened or closed. Furthermore, a disadvantage is involved that at the start of control, as shown in FIG. 2, an overshoot of the control amount with respect to the target value is increased. Conversely, by increasing the width .beta. of the dead zone, the frequency of opening/closing the gate is decreased; however, an operation could be in a stable state in the case where the actual water level comes off the target value, in which case a disadvantage is involved that accuracy of the actual water level with respect to the target value is degraded.

Accordingly, a principal object of the present invention is to provide a feedback automatic control method and apparatus capable of decreasing an overshoot of a control amount with respect to a target value at the start of control and capable of stabilizing a control of a machine to be controlled at the steady state.

According to the present invention, a first dead zone including a target value at the start of control and having a relatively wide width is set and an object being controlled is controlled such that the control value of an object being controlled is brought within the range of the first dead zone. After the lapse of a predetermined time period, a second dead zone having a width narrower than that of the first dead zone is set and the object being controlled is controlled such that the control amount of the object being controlled may be in the range of the second dead zone.

Thus, according to the present invention, since the width of the dead zone is selected to be broader at the start of control, the object being controlled may be simply controlled such that the control amount may be within the range of the dead zone at the start of control, with the result that an overshoot of the control value with respect to the target value can be decreased. Accordingly, an operation frequency of the machine being controlled at the start of control can be decreased. Furthermore, since the present invention is adapted such that the width of the dead zone is selected to be narrower after the lapse of a predetermined time period at the start of control, in a steady state the control amount at that time can be made to correspond with approximately a target value by bringing the control value to be within the range of the dead zone, whereby precision of the control can be enhanced.

In a preferred embodiment of the present invention, the first dead zone of the wide width is supplied to the second comparing means at the start of control and at the same time the target value and the detected control value are compared by the first comparing means, thereby to evaluate the deviation, whereupon the deviation and the first dead zone are compared by the second comparing means, whereby the object being controlled is controlled so that the deviation may be decreased. Based on a time dependent change of a current control amount and the target value, the start time for decreasing the width of the first dead zone and the decreasing time period for decreasing the width of the dead zone for each predetermined width are set in a timer means. When the timer means counts the decrease start time, a decrease of the start of the first dead zone is started, whereupon when an output is obtained from the timer means at every time period, the width of the first dead zone is gradually decreased by each predetermined width until the width of the second dead zone is reached. The second comparing means controls the object being controlled such that deviation may be included within the range of the successively decreasing dead zone.

Therefore, according to the preferred embodiment of the present invention, based on a time dependent change of the current control amount and the target value, the start time for decreasing the width of the first dead zone and the time period for decreasing for every predetermined width can be arbitrarily changed and, if and when a change of the control value is abrupt, a decrease of the width of the first dead zone can be made gradually, while conversely if and when a change of the control amount is gradual, a decrease of the width of the first dead zone can be made abruptly. Thus, according to the preferred embodiment of the present invention, an object being controlled can be controlled in adaptation to a change of the current control amount.

These objects and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

FIG. 1 is a block diagram of a conventional feedback automatic control apparatus;

FIG. 2 is a graph showing a change of the control amount in the conventional feedback automatic control apparatus;

FIG. 3 is a block diagram of one embodiment of the present invention;

FIG. 4 is a flow diagram for explaining an operation of one embodiment of the present invention; and

FIG. 5 is a graph showing a change of the control amount in the FIG. 3 feedback automatic control apparatus.

FIG. 3 is a block diagram of one embodiment of the present invention. The FIG. 3 embodiment comprises a control portion 1 incremented by a microcomputer and further comprises a dead zone setting portion 7 and a timer circuit 8, while the remaining portions may be the same as those shown in FIG. 1. The control portion 1 comprises a central processing unit 11, a read only memory 12, a random access memory 13, and an input/output interface 14. The central processing unit 11 is supplied with the current water level from a water level sensor 4 through the input/output interface 14. The read only memory 12 is provided for storing a program necessary for performing feedback control. The random access memory 13 is provided for storing data concerning the current water level amount supplied from the water level sensor 4 and for storing the data concerning the result of an arithmetic operation. The central processing unit 11 makes an arithmetic operation of the first dead zone having a relatively wide width having the upper limit value contiguous to the target value in accordance with the program stored in the read only memory 12. The read only memory 12 stores in advance the second dead zone having the width .beta. narrower than that of the first dead zone. The dead zone setting portion 7 is provided for storing the first and second dead zones obtained from the control portion 1 and comprises a shift register, for example. The timer circuit 8 is supplied with an operation enable signal from the control portion 1 at the start of control. The timer circuit 8 starts a count operation of the time responsive to the operation enable signal and provides a time output signal to the dead zone setting portion after the lapse of a predetermined time period. If and when the dead zone setting portion 7 is supplied with the time output signal from the timer circuit 8, the dead zone setting portion 7 provides the comparator 5 with the second dead zone having the width .beta. by gradually decreasing the width .alpha. of the first dead zone so far obtained.

FIG. 4 is a flow diagram for explaining an operation of one embodiment of the present invention, and FIG. 5 is a graph showing a change of the control amount in the FIG. 3 feedback automatic control apparatus.

Now referring to FIGS. 3 to 5, a specific operation of one embodiment of the present invention will be described. First at a time t0 at the start of control shown in FIG. 3, the central processing unit 11 provides at the step SP1 the first comparator 2 with a predetermined target value 1a and also provides the timer circuit 8 with the operation enable signal 1b. At the step SP2 the central processing unit 11 is responsive to the detected output from the water level sensor 4 obtained through the input/output interface 14 and the target value 1a to evaluate the width .alpha. of the dead zone at the beginning (the first dead zone), thereby to provide the dead zone setting portion 7 with the evaluated result through the input/output interface 14.

At the step SP3 the first comparator 2 compares the target value 1a and the detected output from the water level sensor 4, thereby to determine whether the current water level amount corresponds with the target value. If and when the current water level amount does not correspond with the target value, a deviation within the current water level amount and the target value is evaluated and the deviation value is applied to the second comparator 5. The second comparator 5 compares the width .alpha. of the first dead zone set in the dead zone setting portion 7 and the deviation, thereby to provide a gate drive circuit 6 with the operation enable signal at the step SP4, if and when the deviation does not exceed the width .alpha. of the dead zone. More specifically, the second comparator 5 provides an operation enable signal such that the gate of the dam is opened if and when the current water level is higher than the target value. As a result, the gate of the dam is opened and the water level is decreased. The control amount at that time, i.e. the opening/closing amount of the gate is increased abruptly as shown in FIG. 5.

The central processing unit 11 evaluates at the step SP5 a time dependent change of the current water level supplied from the water level sensor 4. Then the central processing unit 11 is responsive to the target value 1a, the width .alpha. of the dead zone and the time dependent change of the water level to set timer circuit 8 a start timing t1 to decrease the desired dead zone to decrease the width .alpha. of the dead zone to the width .beta. of the second dead zone, and a decreasing time period in which the width .alpha. of the dead zone is gradually decreased to reach the width .beta. of the dead zone. If and when a time corresponding to the start timing t1 for decreasing the dead zone set in the timer circuit 8 is counted, the output signal is supplied to the dead zone setting portion 7. Accordingly, the dead zone setting portion 7 starts a decrease of the width .alpha. of the first dead zone. More specifically, after the timer circuit 8 provides the above described output signal, each time at a time period corresponding to the decreasing time period set as described previously is counted, the output signal is applied to the dead zone setting portion 7. Each time the output signal is supplied from the timer circuit 8, the dead zone setting portion 7 successively decreases the width .alpha. of the dead zone for every predetermined width. The purpose is to decrease the frequency of the gate being opened or closed by making the period for decreasing the width .alpha. of the dead zone gradual, in the case where the detected water level is abruptly changed, and to shorten the period for decreasing the width .alpha. of the dead zone, if and when the change of the water level is conversely gradual.

The second comparator 5 determines at the step SP6 whether the deviation obtained from the first comparator 2 is in the range of the width .alpha. of the dead zone for gradually decreasing as described above. If and when the deviation is not within the range of the width .alpha. of the dead zone, an operation enable signal is applied at the step SP7 to the gate driving circuit 6 so that the deviation may fall within the range of the width .alpha. of the dead zone. The second comparator 5 causes the operation enable signal to have a zero value at the step SP8, if and when the current water level becomes approximately equal to the target value.

Therefore, according to the embodiment shown, if and when the control amount at the start of control abruptly rises as shown in FIG. 5 so that the same exceeds the width .alpha. of the first dead zone having a relatively wide width, the control amount is decreased; however, since the control amount as decreased at that time is included in the width .alpha. of the dead zone, the control amount as changed does not largely exceed the width .alpha. of the dead zone even if the control amount is again increased. Accordingly, an overshoot of the control amount with respect to the target value at the start of control as shown in FIG. 2 can be decreased and therefore the operation frequency of the machine being controlled can be decreased.

In a steady state after the start of control, the gate is controlled such that the water level at that time may fall within the range of the dead zone of the width .alpha. which is relatively narrow, the water level can be made to correspond with approximately the target value, whereby the accuracy of control can be enhanced. Furthermore, since the time period when the width .beta. of the first dead zone is decreased up to the width .alpha. of the second dead zone can be changed in accordance with the change of the current water level, an object being controlled can be feedback controlled in best adaptation to a change of the water level.

Although the above described embodiment was described as embodied in a feedback automatic control apparatus for opening or closing a gate of a dam, the present invention is not limited to such application and the present invention can be applied to any other types of feedback automatic control apparatus.

Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.